The present invention relates to a monolithic light emitting diode array, more particularly to a monolithic light emitting diode array having light emitting areas surrounded by forward and reverse mesa etching grooves formed so as to be orthogonal to each other.
A light emitting diode array is used, for example, for an electrostatic recording apparatus wherein a photosensitive drum is exposed to light emitted from the light emitting diode array driven in accordance with printing signals.
FIG. 1 shows an electrostatic recording apparatus which comprises: a light exposing means A including a light emitting diode array 1 and a lens array 2 for focussing light emitted from the diode array 1, an image developing means B for attaching magnetic toners to an electrostatic image on a photosensitive drum 3, a transfer corotoron D for transferring the toners on the drum 3 to a recording paper being carried in the direction of the arrow Z, a means E for removing an electrostatic potential of the drum by the exposure of light thereto, a means F for removing the remaining toners so as to clean the surface of the drum 3, a charge corotoron G for charging the surface of the drum 1 with a uniform electrostatic potential, and a means H for fixing the toner image transferred to the recording paper by heating and pressing the toner image on the paper.
FIG. 2 shows a light exposing means A including a light emitting diode array 1 and a lens array 2. The lens array 2 consists of a plurality of rod-like lenses each having self-focussing function of light 2a emitted from the light emitting diode array 1.
In operation, the photosensitive drum 3 rotates in the clockwise direction and the surface thereof is charged uniformly in accordance with the discharge of the charge corotoron G. The charged drum 3 is exposed by light emitted from the diode array 1 driven in accordance with recording signals so as to produce electrostatic image thereon. The electrostatic image on the drum 3 is developed by receiving toners from the developing means B when it passes a contact point with the developing means B in the clockwise rotation thereof. The developed image with toners on the drum 3 is transferred to the recording paper being carried in the direction of arrow Z by electrostatic field produced by the transfer corotoron D. The transferred image on the recording paper is fixed by the heat and pressure of the fixing means H to provide a completely recorded paper. Finally, the electrostatic image of the drum 3 is erased by light exposure of the means E for removing the electrostatic potential of the drum 3 and the remaining toners on the drum 3 are removed by the cleaning means F so as to thereby finish one recording cycle of an electrostatic recording system.
FIGS. 3 to 5 show a monolithic light emitting diode array which is used, for example, for such an electrostatic recording apparatus. The light emitting diode array compress an N-type galium arsenide (hereinafter called "GaAs") substrate 10, an N-type GaAs.sub.1-x P.sub.x layer 11 formed by epitaxial growth (x: a mixed crystal ratio), an N-type GaAs layer 12, P-type light emitting recombination portions 13 each formed through the surface of the N-type GaAs layer 12 by the preferential diffusion of Zn, isolation stripes 14 each formed by the same method used for the formation of the light emitting recombination portions 13 for isolating the respective light emitting recombination portions 13, individual electrodes 15 provided on the respect light emitting portions of the N-type GaAs layer 12 and to which a positive potential is applied, and a common electrode 16 provided on the back surface of the N-type GaAs substrate 10 and to which a negative potential is applied. The N-type GaAs layer 12 and each of the electrodes 15 are provided with an aperture 17 respectively for the extraction of the light emitted beneath the respective light emitting recombination portion 13.
In operation, electrons are injected from the
N-type GaAs.sub.1-x P.sub.x layer 11 to the P-type light emitting recombination portions 13 when a forward bias potential is applied between the respective positive electrodes 15 and the common negative electrode 16 whereby light emitted in accordance with the light emitting recombination is radiated to the outside as shown by the arrow L, from the apertures 17 each of which is provided in the N-type GaAs layer 12 and each of the electrodes 15 respectively.
According to the above mentioned light emitting diode array, the uniform brightness is obtained in accordance with the light extracted from the aperture 17, as shown by solid line in FIG. 6, wherein the points x.sub.1 to x.sub.5 in the distance correspond to those of the light emitting recombination portion 13 and the aperture 17 of FIG. 5. This is reason why the approximately uniform flow of electric current is provided in the area of the points X.sub.1 to X.sub.4 because the electrode 15 extends up to the point X.sub.4. In FIG. 6, the dotted line indicates light energy generated in the light recombination portion 13.
According to the monolithic light emitting diode array mentioned above, however, the following disadvantages have been found in accordance with a study by the inventors. (1) There is a limitation to an increase in the amount of light to be emitted with regard to a relatively wide junction area between the P-N layers for the reason there is a fear that the electrode 15 is subject to breakage when the aperture 17 is made bigger so as to extract a greater amount of light since the remaining portion of the electrode 15 near which the aperture is formed inevitably becomes narrower. (2) The process for manufacturing a light emitting diode array, for instance, including a step of forming diffusion mask is complicated because of the preferential diffusion of Zn is adopted. (3) The costs for the manufacture of a light emitting diode array is relatively high because of the preferential diffusion of Zn. (4) There is a fear that the yield becomes low for the reason why there is a tendency to make an unsuitable diffusion, especially at crystal defect portions because the preferential diffusion of Zn includes an interstatial step. (5) In general, it is desirable for a positive potential to be applied to the common electrode while a negative potential is applied to individual electrodes for driving a light emitting diode array. In this regard, it is said to be disadvantage that the negative potential is applied to the common electrode for the reason that a practical doner diffusion source in a III-V group compound semi-conductor is unavailable so that only an N-type substrate can be utilized. (6) It is rather difficult to manufacture a light emitting diode array of a fine device structure for the reason that the process for wire-bonding is inevitable because the respective electrodes for the respective light emitting portions are wired by leads. (7) The light generated in the light emitting recombination portion tends to leak to other area as shown by arrow Y in FIG. 5 thereby decreasing the quality of the printing image where the light emitting diode array is used, for instance, for an electrostatic recording apparatus.